Current demands for wireless radio frequency transmitters often require both increased radio frequency bandwidth and lower undesired emissions of the transmitters. Wider transmitter bandwidth requires an increase in the corner frequency of a baseband filter of the radio transmitter. However, increasing the corner frequency of the baseband filter is often problematic, as increasing the corner frequency may subsequently lead to increased undesired emissions from the radio transmitter. This increasingly makes meeting undesired emissions requirements much more difficult.
One method to increase bandwidth without increasing undesired emissions is to place the filter corner frequency inside the transmitter bandwidth and correct any resulting droop using digital equalization. This approach requires a very accurate filter frequency calibration because the filter's frequency transfer function must closely match the expected response inverted by the digital equalizer.
Other implementations have focused generally on the tuning of a filter. Such implementations include making direct measurements of a resistor and capacitor on a semiconductor chip. This method, however, is inexact as it does not capture the impact of the active circuits in the filter, such as op-amps, on the frequency response. Another method for fine tuning of a filter involves reconfiguring the filter as an oscillator and measuring the oscillating frequency. This however, requires modifying the filter from its intended configuration, which often leads to the addition of calibration errors.
Another method for fine tuning a filter is directed towards stimulating a filter with a periodic signal (such as, for example, a square wave) and measuring the time it takes to reach a zero crossing. This method for tuning however, in general, is also restricted towards the calibration of a first order filter response and is ineffective for higher order filters. Another method includes stimulating a filter with a sinusoid wave at the corner phase and using a phase detector to measure when the input and output appear to be 90 degrees out of phase. This method is inefficient, as it fails to calibrate the peaking frequency, and requires additional hardware in its implementation.
Thus there remains a need in the art, for an efficient system and method for calibrating the frequency response of an electronic filter with high accuracy, without the need for additional hardware. There also remains a need in the art for calibrating the frequency response of an electronic filter during a standard transmission operating mode.